In a Multiple Input Multiple Output (MIMO) system, the base station transmits signals using multiple antennas, namely, Multi-antenna transmission is implemented at the base station. Multi-antenna transmission may be classified into open-loop and closed-loop according to whether the base station needs feedback information of the down link. An open-loop Multi-antenna transmission refers to that the transmitter sends signals without knowing the channel state in advance, which typically is that each antenna transmits with the same power, the advantage thereof lies in that, the system is more simple, and the performance is not influenced by the channel feedback; while the shortcoming lies in that, the channel information is not fully used. While a closed-loop transmission is that, the channel state of the down link is fed back to the base station by the mobile station, the base station calculates the weighted coefficient of each mobile station according to the feedback information, to optimize the match between the transmitting wave and the channel state. However, the accuracy and the time delay of the feedback of closed-loop method are highly required, if the speed of the channel changing is higher than the feedback speed of the mobile end, the optimal weighted coefficient of the antenna will not be updated in time. Therefore, the most serious challenge faced by the closed-loop MIMO technique is the acquisition of accurate channel state information at the transmitter (CSIT).
The most popular existing feedback solution for the closed-loop MIMO is the codebook-based feedback solution. The codebook-based feedback is not affected by the asymmetry between the uplink channel and the downlink channel, and is applicable for the TDD and FDD system. Another feedback solution that is based on sounding is only applicable for the TDD system, and suffers severely from the asymmetry between the uplink channel and the downlink channel. Therefore, codebook based solution is the most popular feedback solution in the existing system. However, it suffers from two major issues: the large feedback overhead and inaccuracy of CSIT due to feedback quantization, delay, etc.
The spatial correlation of the antenna, namely, the correlation between the channels corresponding to the antenna, is closely related to the scattering transmission (including the scattering objects in the space) and the character of the antenna. The spatial correlation of the antenna also reflects the orientation of the channel, i.e., the level of independency of each sub-channel. The stronger the spatial correlation of the channel is, the more concentrating on its statistic orientation the distribution of the channel array is, thus only the information of the statistic orientation needs to be fed back; on the contrary, if the spatial correlation of the antenna is very weak, the channel array shows no orientation, and the distribution in the space is average, therefore, large quantity of feedback will be needed if the information of the channel whose spatial correlation is very weak needs to be reflected accurately.
Furthermore, in the current solution using cross-polarization, closely spaced cross-polarized antenna elements are applied. On the one hand, the closely placed antenna elements provide strong spatial correlation; on the other hand, each antenna element has two polarization directions that are perpendicular to each other, for example horizontal polarization and vertical polarization, therefore, each antenna element provides un-correlated polarization directions, and may work as two virtual antennas that are spatially un-correlated. By transmitting signals over the two polarization directions, a virtual un-correlated MIMO system can be constructed. However, the disadvantage of this solution is that the achievable degree of freedom of this virtual un-correlated MIMO system is limited to two.